A novel undulator-based PGM beamline for circularly polarised synchrotron radiation at BESSY II

We present the design of a beamline for circularly polarised radiation from an elliptical double undulator to be built at BESSY II. The UE56 undulator of Sasaki type will emit simultaneously two angularly separated circularly polarised beams of opposite helicities. The undulator will, through emissions in first, third and fifth harmonics, provide circularly polarised radiation in the energy range 89–1328 eV. The beamline essentially consists of a plane grating monochromator (PGM) working with collimated light and employing only one set of optical elements for steering and monochromatising the two beams. A high energy resolution from 6000 to 13 000 can be achieved at a flux between about 1010 and 1012 photon/s, respectively. The helicity of circular polarisation can be switched dynamically in the beamline at a high rate (>100 Hz) by use of a suitable chopper. The degree of circular polarisation of the source ranging from 78 to 100% is well preserved by the beamline. In the worst case — at the lowest energy — the degree of polarisation is reduced by 5%. In the higher energy range above 357 eV the input degree of circular polarisation can even be increased by the beamline.